This invention relates to a method of magnetically navigating medical devices in the body with a combination of a magnetic field and a magnetic gradient, and to medical devices adapted therefor.
Magnets have long been used to navigate objects in the body. The earliest use of magnets was for removing magnetic materials, such as steel shrapnel, from a patient""s body. Subsequently, magnets were used to move medical devices in the body. More specifically, magnets have also been used to create a magnetic field in the body for orienting magnetic medical devices, or to create a front field magnetic gradient (i.e., a gradient substantially aligned with the magnetic field) for pulling magnetic medical devices in the body. There are a number of medical applications in which it is desirable to both orient a medical device and apply it against an internal body structure in the selected orientation, for example in cardiac mapping, cardiac pacing, or cardiac ablation procedures. This ability would also be useful in the targeted delivery of therapeutic agents, for example delivery of growth agents for percutaneous myocardio revascularization. While magnetic fields have proven effective for orienting medical devices in the body, and to a lesser extent end field magnetic gradients have proven effective for moving medical devices in the body, its has not been possible to orient a medical device and apply it against an internal body structure in the selected orientation.
The ability to orient and apply a medical device against an internal body structure would have a wide range of medical applications, and particularly in cardiac electrophysiology. There are several types of abnormally rapid heart rhythms, or tachyarrhythmias. Over the past decade, reliable cures for some of these tachyarrhythmias have been developed using catheter-based radio frequency (RF) energy to ablate the piece of cardiac tissue responsible for the arrhythmia. These conditions include Wolfe Parkinson White syndrome and Atrio-Ventricular Nodal Reentrant Tachycardias, where procedural success rates are as high as 99-100%. The catheters used for RF ablation have one or more electrodes at the distal end and typically contain a means for manual mechanical navigation with a heart chamber. The physician uses controls at the proximal end of the catheter to navigate the distal tip to the desired location and to attempt to hold it in constant contact with the heart wall during the application of RF energy.
More recently, much attention has been paid to another cardiac tachyarrhythmia, atrial fibrillation (AF). AF is characterized by a rapid, disorganized beating of both atria. AF affects over 2 million people in the U.S., mostly those over 60. While AF is not acutely life threatening, it can have debilitating symptoms and raises the risk of stoke (via embolized thrombus formed in the stagnant atria) by a factor of 5. The only curative procedure to date for AF is the surgical Maze procedure, developed in the mid-1980""s at Washington University in St. Louis. As described in Cox et al., J Thorac Cardiovasc Surg 1995,110, 473-495, (incorporated herein by reference), several transmural incisions are made in both atria (in a particular anatomic pattern), interrupting the fibrillatory xe2x80x9ccircuitsxe2x80x9d in the tissue while allowing conduction to proceed in a circuitous path throughout the atria.
Substantial work has been done to attempt to mimic the Maze procedure with a less invasive, percutaneous approach. This work has resulted in the development of a number of xe2x80x9clinear lesion catheters.xe2x80x9d These devices are designed to create transmural linear lesions which mimic the surgical incisions of the Maze procedure. While most use an array of RF electrodes (e.g. Avitall, U.S. Pat. No. 5,730,127; Li et al., U.S. Pat. No. 5,879,295; Schaer, U.S. Pat. No. 5,863,291; Chen et al., U.S. Pat. No. 5,782,828; and Pomeranz et al, U.S. Pat. No. 5,800,482, each of which is incorporated herein by reference) other tissue ablation technologies have also been incorporated into these catheters as well, such as cryogenic ablation (e.g., Avitall, U.S. Pat. No. 5,733,280, incorporated herein by reference), and ultrasound (e.g., Crowley, U.S. Pat. No. 5,630,837, incorporated herein by reference). The xe2x80x9ccatheter Mazexe2x80x9d procedure consists of placing a series of interconnected linear lesions in both atria. For this procedure the catheter must be navigated to, and positioned at, several sites in both atria. Clearly, there is an additional constraint on navigating linear lesion catheters that is not imposed on point ablation catheters: orientation. Not only must the catheter be navigated to the proper location and held there, it must be positioned in the proper orientation.
Prior to this invention, linear lesion catheters have been navigated, positioned and held in place manually via mechanical controls at the proximal end of the catheter. The linear lesion catheters designed to date share one or more of the following problems: Most of these catheters are optimized for some locations and orientations, and are sub-optimal for other orientations. The ability to navigate and position the catheter using mechanical controls is dependent on the tortuosity of the catheter""s path to the chamber. The more tortuous the path, less navigational and positional ability remains once the catheter reaches the chamber. The proximal end of the catheter is stationary, while the distal segment is moving with heart chamber, making it difficult to assure good contact of all electrodes, or other lesion making surfaces, throughout the cardiac cycle. For example, if ablation electrodes are not in intimate contact with the walls of the heart, the electrodes can overheat the blood and coagulum can form on the catheter reducing its effectiveness. These limitations have proven particularly problematic when attempting to create lesions in the left atrium.
Also, since the heart wall is moving, the force supplied by traditional catheters to the heart wall varies during the cardiac cycle and the physician cannot quantitatively sense how much force is being exerted against the tissues. As a consequence the mapping and ablating process is not readily reproducible, even for endocardial sites which may be easy to navigate to.
The present invention relates to a method of navigating medical devices in the body which employs both a magnetic field to selectively orient the medical device, and a magnetic gradient to pull the medical device against a selected body structure. Generally the method comprises the steps of applying a magnetic field to the elongate magnetic element to orient the elongate magnetic element in a selected orientation; and applying a magnetic gradient to the elongate magnetic element to draw the elongate magnetic element against the surface of the body structure. The magnetic gradient is preferably oblique to the magnetic field direction, so that the pulling force is at an oblique angle to the aligning force, and more preferably the gradient is perpendicular to the field direction. Thus, the elongate magnetic element is held against the body structure substantially along its length. This can be conveniently done with one or more permanent magnets, using, for example the side field of a single permanent magnet or using the field in the gap of a gapped toroid magnet, but it could also be done with electromagnets, or superconducting electromagnets.
This present invention also relates to a magnetic medical device adapted to be applied to the internal structures of the body. Generally, the medical device includes an elongate magnetic element comprising one or more magnetically responsive bodies. The magnetically responsive bodies can comprise cylinders or coils of a permeable magnetic material or a permanent magnetic material. The magnets are preferably axially polarized so that the elongate magnetic element aligns with an applied magnetic field. The direction of the polarity of the magnet bodies with respect to the longitudinal axis of the elongate magnetic element can vary, to cause the elongate element to bend to facilitate the elongate medical device conforming to the internal body structure to which it is applied. The magnetically responsive bodies could also be spherical magnets, pivotally mounted in the elongate magnetic element so that they can align with the applied magnetic field, still apply an aligning torque to the elongate magnetic element.
The elongate magnetic element can include electrodes for mapping, electrodes for pacing, or electrodescontacts or other elements for tissue ablation. The elongate magnetic element could alternatively or additionally include injectors or simply openings for the delivery of therapeutic or other agents from the medical device, for example growth factors for percutaneous myocardial revasculariation, or simple saline for cooling the electrodes/contacts used in ablation.
The method of the present invention allows an elongate medical device to be precisely oriented and applied to the surface of an internal body structure. This facilitates a number of procedures, including cardiac mapping, cardiac pacing and cardiac ablation. The magnetic field and the oblique magnetic gradient can be easily and economically applied with a single permanent magnet, and thus the method can be employed with relatively inexpensive equipment, although the magnetic field and gradient may be applied with a gapped toroid magnet, or one or more electromagnets or superconducting electromagnets.
These and other features and advantages will be in part apparent, and in part pointed out hereinafter.